1. Field of the Invention
This invention relates to the field of rigid cellular urethane-modified polyisocyanurate compositions and specifically to a catalyst for their preparation.
2. Description of the Prior Art
The preparation of polyisocyanurate foams containing urethane groups is described in numerous publications and patents. Examples thereof are the publications in Journal of Cellular Plastics, January 1965, pages 85-90; Journal of Cellular Plastics, July 1968, pages 248-260; Kunststoffe (Plastics) 62 (1972), pages 731-732 as well as British Application No. 1,155,768.
For the preparation of urethane group-containing polyisocyanurate foams, organic polyisocyanates may be cyclized and/or polymerized in the presence of auxiliaries and catalysts and the resultant isocyanate group-containing polyisocyanurate and/or polymers may subsequently be reacted with polyols. Advantageously, however, the trimerization and/or polymerization of the polyisocyanate and the polyaddition of the polyols to the polyisocyanate or isocyanate group-containing polyisocyanurates are carried out simultaneously.
Suitable catalysts for cyclizing and polymerizing the polyisocyanates include, for example, strong bases such as quaternary ammonium hydroxides, for example benzyltrimethylammonium hydroxide; alkali metal hydroxides such as sodium or potassium hydroxide; alkali metal alkoxides such as sodium methylate and potassium isopropylate; trialkylphosphines such as triethylphosphine; dialkylamino alkylphenols such as 2,4,6 tris(dimethylaminomethyl)phenol; 3- and/or 4-substituted pyridines such as 3- or 4-methylpyridine; metal organic salts such as sodium tetrakis(hydroxyethyl)borate; Friedel-Crafts catalysts such as aluminum chloride, ferric chloride, boron trifluoride, zinc chloride and alkali metal salts of weak organic acids and nitrophenols and imides such as potassium octanoate, potassium 2-ethylhexanoate, potassium benzoate, sodium picrate, and potassium phthalimide. Also used are the strongly basic N,N',N"-tris(dialkylaminoalkyl)-s-hexahydrotriazines such as N,N',N"-tris(dimethylaminopropyl)-s-hexahydrotriazine, optionally combined with aliphatic, low molecular mono and/or dicarboxylic acids such as acetic acid and/or adipic acids, or aromatic carboxylic acids such as benzoic acid.
In addition to the referenced cyclization and polymerization catalysts, catalysts are normally used which accelerate the addition reaction between the polyisocyanates and the polyols. Suitable so-called polyurethane catalysts include, for example, tertiary amines such as dimethylbenzylamine, bis(dimethylaminoethyl)ether, bis-(dimethylaminopropyl)urea, N-methyl and/or N-ethylmorpholine, dimethylpiperazine, 1,2-dimethylimidazole, 1-aza-bicyclo[3,3,0]octane and preferably triethylenediamine, metal salts such as tin dioctanoate, lead octanoate, tin di-2-ethylhexanoate and preferably stannous salts and dibutyltin dilaurate as well as particularly mixtures of tertiary amines and organic tin salts.
In German Application No. 26 31 733 (U.S. Pat. No. 4,040,992) quaternary hydroxyalkylammonium compounds are described as highly effective catalysts for addition and/or polymerization reactions of organic isocyanates for the preparation of urethane and/or isocyanurate group-containing products such as polyurethanes and polyisocyanurates.
Such quaternary hydroxyalkylammonium compounds, however, as do mixtures of polyisocyanurate and polyurethane catalysts have the disadvantage that they undergo the so-called intermittent PIR reaction. Under the designation "intermittent PIR reaction," a description of foaming behavior has been introduced into the literature (for example, G. F. Baumann and W. Dietrich, Journal of Cellular Plastics, May/June (1981), 144 and J. Bechara, Journal of Cellular Plastics, March/April (1979) 102) wherein, corresponding with FIG. 1, the curve of the foaming rate A of foam formulations with an isocyanate to hydroxyl group ratio greater than 2 shows 2 maximum of the foaming rate during the foam formation, or wherein curve B of the specific volume [cubic centimeters/gram] shows 2 points of inflection as a function of time [minutes]. In the curve of the foaming rate A, the first maximum of the foaming rate is correlated with the urethane formation and the second maximum foaming rate is correlated with the isocyanurate formation. The drawback of this mode of operation is that the polyurethane reaction is accelerated first and that the polyisocyanurate reaction is accelerated on a subsequent basis only so that inhomogeneous foam pieces are obtained.